There are imaging devices, which are an example of optical devices, equipped with a lens assembly comprised of a lens and a holder for holding the lens, a circuit board, and a housing that houses the lens assembly and circuit board. In such an imaging device, the occurrence of condensation on the circuit board may cause water droplets to produce water migration in the circuit board, resulting in insulation failures in the circuit board.
There are some methods for reducing the risk of the occurrence of condensation on the circuit board. A first method is to coat a drip-proof material on the circuit board, and a second method is to encapsulate a drying agent or inert gas in the housing. A third method is to design the housing having a closable passage communicating between the inside and outside of the housing, and fit a moisture permeable material impervious to water in the closable passage. The second and third methods are disclosed respectively in Japanese Patent Application Publications No. 2002-221748 and 2008-239017.